1. Field of the Invention
This invention relates to a method for producing a soft-edged aperture filter comprising a glass matrix and having good thermal stability, and to a glass which is particularly suited for the production of such a soft-edged aperture filter.
2. Description of the Prior Art
Generally, when large light beams are truncated at the input edge of a glass rod, a strong focusing of light occurs in the central part of the glass rod due to Fresnel diffraction at the edge of the glass rod. In a laser glass used for emitting high output laser light, this self-focusing effect due to diffraction induces breakage of the glass. In order to prevent such Fresnel diffraction and thus to prevent breakage of the glass, a soft-edged aperture filter is used. The filter is one which has high transmittance within a radius r.sub.o, and whose transmittance gradually drops to a low value at greater radii, and which then prevents transmission of light beyond a certain radii. Such filters are described in detail in A. J. Campillo et al; Laser Focus, June p62 (1974), A. J. Campillo et al; Appl. Phys. Lett., 23 [2] 85 (1973), A. J. Campillo et al; Submitted to VIII International Quantum Electronics Conference (1974) D7, and in A. J. Campillo et al.; Submitted to Opt. Communication.
Conventional methods for producing soft-edged aperture filters include the following. One method comprises shielding the light generated from a plate-like light source and scattered by means of a scattering plate with a circular plate, and printing the light on a silver halide photographic film. The portion which is completely shielded from light by the circular plate becomes transparent after development, but the portion which is exposed to diffraction light from around the circular plate forms a transmittance gradient around the transparent portion after development in which the transmittance gradually decreases at increasing radial distances, i.e., a decrease in transmittance occurs in the region of diffraction light exposure (see the fourth reference above cited by Campillo et al.) On the other hand, the portions exposed to the light not shielded by the circular plate become black after development, and have a transmittance of 0. This method has the advantages of low cost and simplicity, but is defective in that the resulting filter does not have good thermal stability. Furthermore, it is necessary to coat thereon a refractive index matching liquid (e.g., glycerol) to remove interference fringe ascribable to the non-uniform thickness of the silver halide photographic film.
Another method involves the use of a vacuum-deposited metal film, etc., and comprises vacuum-depositing films which have different refractive indices with increasing radius from r.sub.o concentrically in an annular form on a transparent substrate plate, so that the final film does not permit the transmission of light at radius r.sub.1. However, since the number of vacuum-deposited layers is limited, a smooth transmittance gradient from r.sub.o to r.sub.1 can generally not be obtained, and it frequently becomes ripple-like. Moreover, the manufacturing process is complicated, and the resulting filter does not have good thermal stability.